In the past few years, it has proved to be socially and commercially advantageous to perform vibration testing and dynamic analysis on a wide range of mechanical structures. Such testing and analysis is conducted to determine the mechanical resonances and vibration amplitudes of structures in order to assure their functional stability. Knowledge of the mechanical resonances of a structure enhances the economic optimization of product design. Moreover, such knowledge enables the designer to avoid designs which are susceptible to vibrations at the structure's mechanical resonances. Structures which are so susceptible to vibrations at their mechanical resonances are not only subject to failure or malfunction, they may also pose safety problems. For example, vibrations reaching resonances in an automobile can cause severe road handling problems and passenger discomfort. As a further example, such resonance in an aircraft wing could cause the wing to flutter and to fail. Structural dynamic testing determines if such conditions exist, and permits the structure to be improved in ways to avoid harmful vibrations. In recent years such testing has been greatly enhanced by the use of computers employing mathematical algorithms and procedures, such as Fourier analysis. Such tools also enable the computer to graphically illustrate all of the structure's vibrational motions with an animated visual display.
In conducting such sophisticated tests, a structure is defined for the computer as a series of interconnected points, with each point representing an individual test point on the structure. It is not uncommon for more than a hundred test points to be so defined. At these points on the physical structure, either an excitation force is input, or a motion is detected. In any event, a pair of readings is always taken, an input force and an output motion between the test point and a reference point on the structure. The ratio of the time or frequency representations of the two signals constitute a "transfer function" between the two points. These transfer functions are the basis of the computer analysis and animation.
There are several ways to excite structures for such tests, each having its particular suitability. The two most popular classes of excitation are "shakers", hydraulically or electrically powered machines capable of producing various vibratory forces, and "impactors" which produce impulse forces. Shakers find utility where great amounts of energy must be imparted to a structure. In practice, shakers usually are placed at the reference point on the structure being tested. The companion motion sensor must then be mounted at each test point in order to establish a transfer function for that test point. Excitation of a broad band of vibratory motion is time consuming using shakers. Considerable time expenditure is required for both the placement of motion sensors and for driving the structure through a wide range of frequencies.
Testing with impactors on the other hand, requires substantially less energy and requires substantially less time and set-up costs. Additionally, impact testing inherently produces a broad frequency band of excitation. When testing is conducted using an impactor, the motion sensor is affixed to the reference point, and the impactor is used to simply strike each of the test points.
Prior art impactors have consisted of hammers and similar manual devices with which a sharp blow can be manually imparted to the structure. A force transducer is mounted on the striking surface of such hammers to generate an electrical signal representative of the impact force, which representative signal is processed by a signal acquisition system. A great deal of physical skill and dexterity is required in the use of the manual prior art devices to assure that a proper hit or impact is achieved. Missing the target point, hitting too hard or too softly, creating more than one impact on a single stroke are all errors that commonly occur until significant operator skill is developed. Furthermore, existing manual impact devices for this purpose require an external source of power for the transducer as well as external conditioning of the signals generated by the transducer. Often, this external signal conditioning occurs over long electrical leads. Since the determination of a valid signal is only accomplished at the computer, which is typically remote from the test structure, the long electrical leads represent a substantial source of electrical noise and error. Such prior art manual impact devices are also inherently restricted to environments which are not harmful to the operator. Further, such prior art devices are restricted to applications where there is adequate space for a hammer to be physically swung.